Change 147102 had a related patch set uploaded by Vogone:
Added namespace aliases for thwiki

https://gerrit.wikimedia.org/r/147102

(In reply to Sorawee Porncharoenwase from comment #0)

Also, currently pages with the prefix "WP:" are in main namespace. After the
settings are changed, will these pages be changed to Wikipedia namespace
automatically. In case they will not, how can we rename them?

I believe it is better to move them manually to the วิกิพีเดีย namespace before the change gets merged.

There don't seem to be any pages left in https://th.wikipedia.org/wiki/Special:PrefixIndex/WP: anymore, so I believe this can be safely merged now.

Change 147102 merged by jenkins-bot:
Added namespace aliases for thwiki

https://gerrit.wikimedia.org/r/147102

@Vogone: Yep, I used my bot to move those pages from "WP:" to "วิกิพีเดีย:".

I forgot to move pages with prefix "H:" to "Help:", though, and that makes "H:IL", "H:ILL", "H:IW", "H:L" remains on main namespace[1], unable to access anymore. It is completely my fault. I'm so sorry. Now I just want to know what should I do next to delete these pages from main namespace?

[1] - https://th.wikipedia.org/wiki/Special:PrefixIndex/H

Reedy might help you.

I found that I can use API to delete those pages.