Not lock-in in the traditional sense where you're locked to a particular technology, but effectively lock-in by making a commonly used feature for migration not available for free. This wasn't discussing uncommon cases like having your own domain in front of a free email service since that's not then fully free.

The most common use of free email services is to use the service's domain and if you need to switch, then needing to change your email at tons of different places. I am still stuck on gmail for a couple of accounts because changing the email with those services means creating an entirely new account and thus losing all history, etc. Example is the Shop app. Without forwarding I'd end up having to keep the gmail app on my phone to get notification of new emails which is problematic since those apps come with additional tracking services which is the whole reason for migrating from gmail.