The PlayStation 5 is going to be running SSD internal storage to provide the best experience for all of the new next-gen games, so you are going to want to maximize the space on that drive.
This is where external storage options come in, which have been a great option for players who like having the extra space to keep more games installed and ready to play without having to delete some of their older apps or games to make room.
Unfortunately, Sony has confirmed that some SSD storage will not be supported until sometime post-launch due to various reasons.
This really only translates to the dedicated internal slot the console has that should fit an M.2 SSD stick that can expand the storage without really being an external source. Sony confirmed this internal slot would be disabled at launch, but can be activated in a future update, according to The Verge.
Mark Cerny, lead designer for the PS5, said something similar to this would be the case very early on after the console was announced.
Nothing about this information should limit your use of an external SSD storage if you want to try one out, but there might be some slight drop in performance for PS5 titles running on that external drive compared to the internal drive, so you might be better off just managing the base storage.
And considering the PS5 will likely only have 664GB of usable internal storage at launch, this could be an issue for anyone wanting to download more than just a handful of games.
Sony actually recommends that players utilize an external HDD storage device if they are going to transfer over PS4 titles or download them on their PS5 in any way. That will give you more space on the internal SSD to keep the PS5 games that will benefit the most from the hardware.
PS4 titles being stored on an external HDD will still see improved load times and performance on PS5.