In golang.org/x repo, TryBots testing CLs on branches other than the default "master" branch are only tested with Go tip, not with the past 2 releases (e.g., 1.16 and 1.15).

The gopls-release-branch.* branches in x/tools are used for issuing gopls releases, which are expected to work on currently supported Go releases, so these branches should be included in the wider testing.

(This doesn't apply more generally, e.g., custom branches like "dev.link" or "dev.go2go" may not work with non-tip versions of Go. So start by opting-in for branches that are known to support past Go versions.)

CC @golang/release, @stamblerre, @findleyr.