After updating to Sonoma 4.5 I was no more able to check-spell text-files.
Although my custom file was in ~/Library/Spelling, it was not seen by any app.
Then I came to know that the new location is
~/Library/Group Containers/group.com.apple.AppleSpell/Library/Spelling/
So if anybody finds her/himself in such situation, move your custon file there and reboot.
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