I removed it from bb.org already https://github.com/MariaDB/buildbot/pull/20

It was already announced in the last release notes that there would be no more Debian 9 releases.

Debian 9 'stretch' removed from buildbot.askmonty.org config.

elenst I modified the kvm-sched-devtrees-10.2+ trigger in the config to use buster instead of stretch. This trigger should probably be moved/renamed to build 10.3+ now that both stretch and 10.2 are deprecated. Feel free to make the change yourself, or I am happy to do it, but wanted to let you decide since you were the one that initially created the trigger.

It shouldn't be needed anymore. devtrees scheduler was added at the moment when we had 5.5 and 10.0-10.2, and there was no common deb builder which would build all of them at once – some were too new for 5.5, and some too old for 10.2.
Now since buster builds all of 10.3-10.10, we can just add it to alltrees and drop devtrees. I will do it, but since a change in schedulers usually requires buildbot restart, probably it will have to wait till the weekend. There is no rush.

Stretch removed from the various repository/tarball/etc... prep scripts

Stretch deprecated in repository setup scripts. The scripts (which will be updated at the time of the next releases) will display an error or warning depending on whether repository EOL checking is disabled or not, just like we do for other EOL distributions like Ubuntu Xenial.