I'm going to go even farther back than Bruce here and claim that the last big paradigm shift was Rule 4.0 in summer of 2019. 
There have been additions and changes in Rule 4.1, 5.0, and 5.1, but none should require re-learning anything. (These have been quite helpful changes for many users, but, again, you can just totally ignore the features--like required expressions, sticky triggers, etc.--and continue as before if you want.)
If you have questions, have you looked at the documentation? Bruce is good about posting when changes happen. Here are some examples:
- Sticky Triggers and Other New Rule 5.1 Features
- Restriction Options for Basic Rule, Simple Automation Rule and Rule 5.1
- New App Features in 2.2.8 (covers Rule 5.0 and the new parent Rule Machine app, including hub variables replacing global variables)
These are generally linked to in the release notes, which are a good idea to read before upgrading. The forum is generally better than the notification in the admin UI: Release Notes - Hubitat (HINT: use the bell icon and select "Watching" to get notifications of new posts here, and possibly emails, depending on your forum settings)
The Hubitat Documentation is also a good resource, or will be once it's updated (the newer 5.1 features haven't made their way there yet, hence my linking to the forum posts instead).
