Basically- legacy_member_templates set to 'n' the member profile trigger setting in the cp disappears. HOWEVER the trigger was still active and hitting it sends you to the member profile template with a message they are disabled. It doesn't help that some member tags use them as defaults.

Case in point- validation error logging got the

'This page is not accessible because legacy member templates are not enabled.'

Another case- forgotten password- if you don't set the return it goes to the member template reset - and throws the 'legacy not enabled' error.

Needs testing- plus scan says:

There are still several member-tag paths that default to legacy template elements.

{exp:member:login} path:
{exp:member:forgot_password} path:
{exp:member:reset_password} path:

Registration/member profile related tag methods with “empty tagdata => legacy fallback when enabled”:

The safest modern tags are the explicit *_form variants (login_form, forgot_password_form, reset_password_form, registration_form with tagdata).

SO need to go through the docs and ponder whether this is actually a concern.