This isn’t much of a new change. Support for this type of lists was dropped in Sep 2019 in Pi-hole V4.3.2.
Your lists likely haven’t been loading properly ever since then, if you have been using these lists. What has changed in Pi-hole V5 is that the gravity script tells you how many domains from the list were rejected.
In V4 of Pi-hole, this is the section of the gravity output when it gets to that easylist. The V5.0 gravity output has more detail.
[i] Target: easylist.to (easylist.txt)
[✓] Status: Retrieval successful
[✗] Format: Adblock (list type not supported)
Let’s take an example of a single line from the easylist that is being mostly rejected:
How do you think this should be parsed and converted into something Pi-hole can use (a single domain)? What’s the domain here that is being blocked?
Trying to import lists that are written for an entirely different form of adblocker is pointless. This is like putting gasoline in your diesel automobile and being surprised that the vehicle won’t run.