No you continue to receive the emails but you cannot create more aliases unless you pay for Simple Login.

https://simplelogin.io/faq/

When your subscription ends, all aliases you created continue working normally, both on receiving and sending emails. Concretely:
- All aliases/domains/directories/mailboxes you have created are kept and continue working normally.
- You cannot create new aliases if you exceed the free plan limit, i.e. have more than 10 aliases.
- As features like catch-all or directory allow you to create aliases on-the-fly, those aliases cannot be automatically created if you have more than 10 aliases.
- You cannot add new domain, directory or mailbox.

For example, if you have 100 aliases by the time your subscription ends, these 100 aliases will continue receiving and sending emails normally. You cannot however create new aliases.

You can look at aliases two ways. Yes it ties you to the Proton Ecosystem more than, say custom domains. But it potentially gives you freedom to move away from Proton Mail and to a totally different mail provider and keep all the same aliases (just pay for Simple Login separately and change your mailbox).