It is not only for expansions but general during primetime and times of high load of the servers.
When Worlds reach a certain threshold of logged in characters they shut down the character creation automatically.
For years the general advise for newcomers and creating a new character on more crowded Worlds is "Do it during the low times, early morning before going to work or very late at night".

Now that Worlds are constantly at or over that certain threshold there is no way to create a new character during prime time at all and I really recommend to wait for the low times which is generally around 01:00 AM and 06:00 AM of the local timezone the server is in (GMT for EU, CST for US and JST for JP datacenters) or during the prime work times of that time zones.

For planing to do something about it is easier said than done. Upgrading the servers with additional hardware needs hardware to be available and affordable. If you order some high performance servers you will sit in a queue not unlike our login queue right now but with a destination time of like 1 year. But you can circumvent the queue for the server if you take money in your hands, much money, not millions of $ but billions of $ and that is no longer the weight class SE is in.
All they can do right now is tinkering with their adjusting screws and hoping they don't sacrifice to much stability for capacity.