It’s enabled by default (not only for new forums but also for existing) but you can still use the discourse_reactions_enabled site setting and turn it off after updating. You can still opt out.
I am not sure this is going to work for two reasons. I think being on ESR will mean that you miss upcoming changes that move through all stages before the next ESR release happens. Simplified category setup is an example for that. While admins updating each month had the chance to try this and provide feedback, you just get the result like any other code change without an upcoming change.
Also, you can opt-in to changes more early or opt-out after it was enabled automatically when it’s promoted to beta - but you cannot prevent that it’s automatically enabled. (But I don’t think this is a problem in your case)
Of course, checking out changes in alpha to be aware of what’s coming makes sense. You can already test the feature as an admin without it affecting your users and you can quickly opt-out after you performed an update that enabled it - but since you are on ESR there won’t be an update that promotes the upcoming change. For you nothing will promote itself until your update to a later version, but this also means you miss the option to try before stuff is changed permanently or you can try how it’s today, but won’t be able to try the improvements happening in further iterations.