This plugin is now bundled with Discourse core and no longer needs to be installed. Next time you run a ./launcher rebuild app or update from Admin UI it will fail with the error:
bootstrap failed with exit code 128
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HINT: The plugin 'discourse-reactions' is now bundled with Discourse and should not be included in your container configuration.
Remove the line 'git clone https://github.com/discourse/discourse-reactions' from your containers/app.yml file, then try again.
For more information, see https://meta.discourse.org/t/373574
Ideally remove this plugin from your app.yml now to prevent problems when updating.
The same thing happened to me today when I updated. Not just reactions but quite a few plugins have now been bundled with core Discourse and so need to be removed from app.yml.
I’m not sure what the problem is here given that the error is very clear. I was surprised too but followed the directions in the error to remove the offending items and then rebuilt, and all was well.
I also thought the category chosen for the announcement was rather unusual. I don’t think anyone watches it because support questions are often asked there.
Maybe subcategories in announcements would be better, then you could inform hosting customers and self-hosters (and devs) separately. Everyone is informed about a release, but you can choose whether you want to be informed about all subcategories.
Also, a reply to the topic to bump it after the edit would have been helpful. Otherwise, it’s very easy to miss edits.
It’s not a huge problem, but it sucks to find out there’s a problem when doing a routine rebuild, eg for the next version upgrade.
For breaking changes in Discourse Core that affect theme components, we get advisories visible to Admins only for weeks in advance, allowing us to go and fix or remove those theme components. The same would be great for when it’s planned that a plugin will become Core.
For example, I know from this and and linked Topics that Data Explorer is going to be added into Core. That’s great. But I need a decent advisory/announcement on when that happens because I will remove it only when it’s included. Otherwise if I remove it now it risks there being a period of time withoutData Explorer.
I decided to move this recent discussion from the reactions plugin topic to Site feedback, since it is about how we use this site to communicate about plugins moving to core. This will happen from time to time but now we’re doing a big push to move many popular plugins into core, which will affect many self-hosters. This is announced in Bundling more popular plugins with Discourse core.
@moin suggested the Bundling more popular plugins with Discourse core announcement topic could have been posted to Announcements, or we could create a sub-category there for announcements for self-hosters. That’s a good idea too. But then we’d have to be mindful not to confuse our self-serve and enterprise hosting customers who do not need to do anything.
All the affected plugins need to have their topics reviewed to remove the installation instructions and to indicate they are included in core and do not need to be installed, and given the included-in-core tag.
@JammyDodger can you remind me what the intention is behind the included-in-core tag? Is it as self-explanatory as just to communicate the fact that a given plugin is in core and does not need to be installed in the usual way by adding it to app.yml?
Yeah, it’s just a way to group those ones together and indicate that they’re the ‘bundled’ plugins. And it also serves as a marker for anyone who has the tag as Watching First Post so they get a heads up about any new ones.
So adding the tag, updating the install blurb, and leaving a fresh post to the plugin topic to say ‘good news everyone this is now bundled and doesn’t need to be installed separately, yada yada, remove from app.yml’ is usually the meta portion of things. (Maybe a slightly different flavour if something has genuinely moved into core rather than just bundled as a plugin). Getting that done as close to when it happens is the aim. Hopefully an admin dashboard message is also done as well, but that’s another thing.
For a fuller picture, there are still some inconsistencies with how bundled plugins are presented here on meta and in the UI. Some don’t have plugin topics (polls, Discobot, hide details, local dates, etc), and some are hidden from the plugin page on sites as well (chat, polls, Discobot, and so on). Smoothing those out would be nice so it’s more easily understandable for anyone new to know what’s what.
OK! I did the thing. Added the included-in-core tag, updated the OP, and added a reply. In some cases I found some updates that were needed to the OP too. Thanks @moin for your eagle eye to find problems.
Feel free to flag affected topics that need further fixing or updates. Otherwise I think we are caught up.
this is now bundled and doesn’t need to be installed separately, yada yada, remove from app.yml’ is usually the meta portion of things. (Maybe a slightly different flavour if something has genuinely moved into core rather than just bundled as a plugin). Getting that done as close to when it happens is the aim. 
Hopefully an admin dashboard message is also done as well, but that’s another thing.
eagle eye to find problems.