On Apple Developer Program:

While working on Saylify, I realized that many people around me use Apple devices, and in general more than 20% of all users are on macOS or iOS. On top of that, it is the most solvent audience.

So limiting OAuth to Google and email means restricting people from quick sign-up and login. And in general, by its very idea Saylify can easily become a mobile app, which makes an Apple Developer Account essential.

I decided to register it under Improvy OÜ, and here is what I ran into:

1. The marketing name used everywhere is 'Improvy OÜ' (with the legal form OÜ). Apple only supports the Latin alphabet, which means no umlauts at all.
2. The account verification process is fully manual, and I had to provide my documents, an extract from the business register confirming that the company actually exists, and the most curious part: a letter whose text is literally the following:
'I, Roman K., founder and owner of Improvy OÜ, confirm that Roman K. has the right to act on behalf of the company and carry out the company registration process in Apple Developer Program. Date-signature.'
It sounds like a very subtle bureaucratic joke, but Apple is completely serious.

The review of the application took 3 weeks, and finally I recently received an email: 'Welcome to the Apple Developer Program. Please provide your payment method to complete your enrollment.'

Yes, an Apple developer account is paid and costs $99 per year. Without it you cannot publish an app or even get a token for OAuth.