drop compatibility mode

[oliver-sanders]

Cylc 7 is now long dead, it's time to schedule the removal of compatibility mode.

We should pick a future Cylc 8 version to withdraw support (e.g. 8.7.0 or 8.8.0) and include this in the deprecation notice. We should also announce this on Discourse.

We should also

See also (would make sense to do these at the same time):

• drop python 2 compat cylc-rose#374
• jinja2: bump version #6850

[hjoliver]

I think it's OK to drop compat mode at 8.6. Sites with some users still running Cylc 7 suites ought to have earlier versions of Cylc 8 installed as well, to handle initial runs in compat mode.

[oliver-sanders]

I tagged the issue against 8.6.0 to make sure we announce the version that support will be withdrawn at by then. But I think 8.6.0 is a bit too soon as we hope it will follow shortly after 8.5.0.

We're thinking 8.7.0 (likely late spring 2026 going by our track record) which should give users at least 6 months pre-warning which is pretty reasonable.

[hjoliver]

OK, I'm easy either way. But should probably tag the issue against 8.7 in that case (it says "drop compat mode" not "announce we'll drop it next year").

[oliver-sanders]

Ok, I'll take that as an agreement to remove support at 8.7.0 (let me know otherwise) and add that to the warning now (8.5.0).

... done, please scream if you would like to change this.