Was this discussed with the community?

Is it really necessary? The community is me and another guy. It's a change that has 0 negative side-effects, and IMO behavior that already should have been a default setting in MediaWiki.

Yes. See https://meta.wikimedia.org/wiki/Requesting_wiki_configuration_changes. I know your community is very small but at least an announcement should be made. But...fascinating is that I can see this IS really default state. See

'wgNamespacesWithSubpages' => [
        'default' => [
                 (redacted for briefness)
                NS_TEMPLATE /* 10 */ => true,
                NS_TEMPLATE_TALK /* 11 */ => true,
                 (redacted for briefness)

in the config file at https://noc.wikimedia.org/conf/highlight.php?file=InitialiseSettings.php. So don't know why it doesn't work for your wiki (feel free to delete my tests, my account is Martin Urbanec).

In that case this should be considered a bug, not a site request, right? ;-)

Change 362267 had a related patch set uploaded (by MarcoAurelio; owner: MarcoAurelio):
[operations/mediawiki-config@master] Add 'WP' namespace alias to ruwiki

https://gerrit.wikimedia.org/r/362267

Sorry, wrong ticket entered in the patch I submitted above. Forget about it.

This happened to me and we resolved it removing the wiki entry from IS.php. I'll do this.

Change 362272 had a related patch set uploaded (by MarcoAurelio; owner: MarcoAurelio):
[operations/mediawiki-config@master] Fix nowikisource template namespace subpages

https://gerrit.wikimedia.org/r/362272

See T1335.

Change 362272 merged by jenkins-bot:
[operations/mediawiki-config@master] Fix nowikisource template namespace subpages

https://gerrit.wikimedia.org/r/362272