Slugs: numeric URLs are not allowed

[aristath]

Description

While trying to create a 404 template in an FSE theme from the dashboard I came across an interesting issue:
If I use 404 as the title, the slug became custom_slug_60194c2e24403.
When trying to change the slug to any numeric value I get an error: Updating failed. Invalid parameter(s): slug
Apparently numeric values are not allowed for slugs. Changing it to 404a works, any random number doesn't.

Step-by-step reproduction instructions

• Using an FSE theme try to create a 404 template from Appearance -> Templates.

If I try this on a post (using postname as permalink structure) and use 8888888888 as slug, it changes to 8888888888-2 which is acceptable since it adds the dash.

WordPress information

• WordPress version: 5.6.
• Gutenberg version: 9.9
• Are all plugins except Gutenberg deactivated? Yes
• Are you using a default theme Doesn't matter, any FSE theme will do.

Device information

• Device: Desktop
• Operating system: Ubuntu
• Browser: Firefox

[Mamaduka]

This should be fixed now. See #37579.