Because Okta just acquired Auth0. If we look at previous acquisitions, the usual flow is something like this:

1) Company A acquires company B, promises nothing will change

2) Company A hints that company B will slowly be integrated into Company A

3) Everything Company B did becomes deprecated and migration plans are made for 70% of features to get integrated into Company A

4) Users who need the rest of 30% need to find a different service when things start to get turned off

I see no reason why this Okta/Auth0 acquisition would be different. Especially anything that is branded as Auth0 will disappear or get renamed to Okta. So at least the domain will change, which will require both infrastructure, backend and frontend changes most likely. Sucks, as the change is not needed and doesn't improve anything, it's just needed for Okta to rebrand Auth0.

There are companies that keep purchased brands for years and years. Costs may go up as it becomes "legacy", but corporations do what makes sense.

To assume any product/brand is going to exist indefinitely is unrealistic unless you as a customer have that in your contract (and even then mergers break this routinely).