I propose the following version support changes for Mongoid 8.0:

• Drop Rails 5.1 and earlier, keep 5.2+
• Drop Ruby 2.5, keep 2.6+

Note that Rails 5.1 went EOL in 25 Aug 2019, and Ruby 2.5 is EOL as of 31 Mar 2021 (Ruby 2.6 is also already EOL)

https://endoflife.date/rails

https://endoflife.date/ruby

Given the ease of upgrading Rails 5.x --> Rails 5.2 and Ruby 2.5 --> 2.6, and the fact that the upgrades can be done seamlessly on Mongoid 7.4, I think this will have minimal impact on users. (Speaking as a user myself who has done these upgrades.)