Yes that’s correct, it’s only available on our Enterprise tier hosting plan.
On discourse.org hosting, custom plugins aren’t allows on hosting tiers below Enterprise, so this wouldn’t be an option either.
Plugins are far-reaching into Discourse code and can do just about anything, so we have to restrict the ones used on our servers to maintain security and performance.
If the Salesforce plugin is a must-have and Enterprise is beyond your budget, it may be worth investigating self-hosting on your own servers… this grants you full control of which plugins can be installed.