I looked into this some time ago. To create public payment apps you can only gain access by invitation, and forum responses still say access hasn’t been opened to new members.
This thread shed a bit more light on the process:
Invite to Payments Partner - How? - Partner Discussions - Shopify Developer Community Forums
Because I had clients, I had to develop a payment extension for Shopify Plus merchants as a temporary workaround; it solved the immediate need but isn’t ideal.
It would be helpful if someone from Shopify staff addressed this, since there’s been demand from developers interested in building new payment apps since 2023.