What’s Mongodb’s release cadence, how often is a new major version (potentially requiring a new track) released? is this documented somewhere by upstream?
Is there some commitment from upstream on maintenance of old versions? e.g. is 4.x still supported with security updates? will it continue to be supported now that 5.x is out, and for how long?
Are new versions backwards-incompatible? meaning, if I was running 4.x and install 5.x, will that just work, or do I need to migrate my data/configuration, or will things break horribly?
Major versions that are supported for on-prem are released on a yearly cadence
Major versions are supported for ~3 years. Look here
Yes, upgrading versions can be a bit messy as technically you have to be on the latest minor release to upgrade to the next major which afaik we have no plans to support minor versions.
edit: #3 may be partially untrue. For newer versions of mongo, you dont have to be on the latest minor as shown here, but i dont think theres any guarantee there will be a mess-free upgrade between all versions