I don't know if anyone is still curious about Switch DLC region locked or not (I am at least lol) but here is what I found. If you press (+) on your joy-con or controller, it will boot up a software information page, under the "Software Information" tab on the left, there is a "Support Information" option, select that and it should boot up a page that contains all the support region for your specific game and its cartridge. So if you are worried about DLC compatibility or if you bought an overseas copy of a game, you can use this method to check whether your home region or country is supported. Here are some examples just incase it wasn't clear enough (these are from my collection of cartridges so feel free to check yours in case there are differences):
Pokemon Sword/Shield: USA/Canada/Latin America/Caribbean, Europe, Australia/NZ, Japan, Hong Kong/Taiwan and Korea are all supported, therefore, any DLC related to these games will compatible with cartridges from any region and vice versa.
Fate/EXTELLA Link: USA/Canada/Latin America/Caribbean region only therefore only DLCs purchased in the American region is compatible. (I bought the US version of the game because my local game shop ran out of copies so they imported from the US so different regions will have different compatibility)
Splatoon 2: USA/Canada/Latin America/Caribbean and Australia/NZ so only DLCs from these 2 regions are compatible. Different cases for Europe and Japan.
Now, here is a special case. Sometimes you may find your home region missing on the information page. Fear not, you didn't buy an incorrect copy, its just that sometimes that developers "shortened" the information on regions. Example:
Atelier Ryza: US and EU, so in this case EU includes regions such as Australia and South Africa and US should be self-explanatory. I am not too sure about the Japanese version of the game since my copy is from Australia. To keep it simple, you may refer to the PAL, SECAM and NTSC regions and what countries they cover.
Just a safety advice if you have the same/similar informations displayed as my copy of Atelier Ryza, ONLY buy foreign DLCs if 1. you bought you own country's version of the game and 2. if your region doesn't show on the support page. This is just incase of incompatibility, so taking my copy of Atelier Ryza for example, I wouldn't buy a Japanese DLC because the region wasn't listed on the support page. Just a final heads up, sometimes regions are not listed but instead there is a support url, identify any hints to a region within the url as that may point to the supported regions of the game.
