For clarification, it is enabling Flow by default on user talk namespace, for pages which does not exist, without moving existing talk pages.

@Vriullop Is it okay to make all non-existent pages in the namespace Flow?

Technically, that is not quite the same as "of new users" (e.g. if an older user didn't have a user talk page for some reason, or deleted it, it would now be a non-existent/ready Flow board).

@Mattflaschen-WMF since I think most (if not all) new users were receiving a welcome message in their talk pages, I imagine there must be little difference, and @Vriullop suggestion would be OK (a slightly larger subset of pages) as far as it is technically viable.

It looks from https://ca.wikipedia.org/w/index.php?title=Usuari_Discussi%C3%B3:Kano_Kano&diff=18196140&oldid=18196131 like you're not using bots for welcomes, so it should keep working (but you may need to change the template not to mention the name due to some issues (T109373: Flow's usage of the page name magic words is different from wikitext talk pages).

If you are using a bot or user script, it needs to be updated (doesn't look like you're using NewUserMessage).

@Vriullop Is it okay to make all non-existent pages in the namespace Flow?

It is okey. Initial motivation was to enable Flow for newcomers. Conclusion of discussion is to enable it by default for new created user talk pages, either of new or old registered users. It is expected that beta feature "Flow on user talk" will continue available as usual and in last intance is up to the user to decide his preferred content model.

In T159482#3069819, @Mattflaschen-WMF wrote:

you may need to change the template not to mention the name due to some issues

I am aware of the problem of page name magic words https://ca.wikipedia.org/w/index.php?title=Tema:Tm0g7zkz2czo8xs6&action=compare-post-revisions&topic_newRevision=tm0g8udvy3llobch. All message templates targeted for used talk will be reviewed. I think it is not a major problem to avoid the user name while there is not an alternative magic word. Currently there is no welcome bot on ca.wiki.

In T159482#3070127, @Vriullop wrote:

It is expected that beta feature "Flow on user talk" will continue available as usual and in last intance is up to the user to decide his preferred content model.

I don't think this is supported currently, so this is blocked on T149897: Decide on any changes to how preferences should work for Flow user talk.

If this is going forward I strongly suggest that T150572: When a page move is triggered by RenameUserJob, page should be moved anyways even if it is a Flow board is worked on. Catalan Wikipedia is a wiki where rename requests can be frequent and the risk of having Flow boards not moving while renaming is certainly high, and it is a pain resolving them. That should also work for the system account "Global rename script" for when the renamer's account is not there. Thanks.

(I feel the dependency is the other way around. I've changed the blocking task mentioned in the top of the description from subtask to parent task; sorry if my assumption was incorrect.)

Subtask is correct here. See https://www.mediawiki.org/wiki/Phabricator/Project_management#Tasks_that_cannot_be_worked_on_yet .