The following is a curated list of upcoming breaking changes for 3.0.0 - we can pin the issue so people can see before/after the major release.

• drop python3.6 support feat: remove support for Python 3.6, require 3.7 or higher #1705
• default to gitlab.com if no URL given (feat(api): default to gitlab.com if no URL given #1278)
• allow options from CLI args or environment variables (feat(cli): allow options from CLI args or environment variables #1296)
• raise error if there is a 301/302 redirection (fix: raise error if there is a 301/302 redirection #1486) (Not likely a breaking changing, but doesn't hurt to wait until 3.0.0)
• Clean up constants defined in gitlab.v4.objects and keep only gitlab.const (refactor(objects): remove deprecated constants defined in objects #1593)
• remove mr.pipelines() method (we have mr.pipelines.list() now) chore(objects): remove deprecated pipelines() method #1592
• remove members.all() method (we have members_all.list() now) refactor(objects): remove deprecated members.all() method #1594
• remove issuesstatistics in favor of issues_statistics (slightly more readable) refactor(objects): remove deprecated project.issuesstatistics #1595
• drop deprecated API endpoints that confuse users (branch.protect) refactor(objects): remove deprecated branch protect methods #1598
• drop deprecated release notes via tags API endpoint (refactor(objects): remove deprecated tag release API #1596)
• switch from master to main (chore: rename master branch to main #1629)
• stop URL-encoding . in path segments (fix: stop encoding '.' to '%2E' #1766)
• do not require config-file to run CLI (feat(cli): do not require config file to run CLI #1743)
• rename confusing to_project_id argument (fix(objects): rename confusing to_project_id argument #1599)
• replace deprecated name_regex attribute in delete_in_bulk() (fix(api): replace deprecated name_regex attribute in delete_in_bulk() #1536)

Any more ideas? Just edit the description if anything's missing!