I have seen sites in both Japan and China limit signups to not only mobile-only provider-specific email domains, but also IP block everything but domestic mobile traffic as well.

Most of those services (mixi was one in particular I remember doing this) stopped this practice close to 20 years ago though, but some still remain.

I think it was partly due to the "Galapagos phone" era of pre-smartphones, where each carrier used slightly different mobile web standards (think WAP and custom emoji).

Here is a current help page for Mitsubishi UFJ bank that lists approved domains for both desktop and mobile use: https://faq.cr.mufg.jp/mufgcard/detail?id=4402

Here is a user complaining (with screenshot) that the mail magazine for tabelog only supports mobile provider email domains: https://king.mineo.jp/reports/13368

Another recent one is the login page for gravity.place has a dropdown for country codes (for mobile login) with only 6 options.