I created a wallet using Electrum for Windows and use the Option to "Extend this seed with custom words." I used a word with a capital letter.
When I tried to restore this wallet using the Electrum Android app there is no way to enter capital letters, only lowercase. Does this mean my custom seed will only work on the Windows version?
Nope! you can use it on mobile, the reason why you're seeing the lowercase is that the keyboard for inputing the seed phrases is inbuilt for the app and normally it suppose to be a lower case in the first place.
Hence, if you try same thing on Windows and you entered a correct seedphrase, try changing some of the seed word to maybe Capitalizing a word/word or changing it to uppercase, it will notify of a valid segwit (if segwit) but it won't support the seed and won't allow you to proceed.
Thus, the custom word is not case sensitive and if you try using that on mobile( you can do this by checking the custom word section below), doing you should be able to access your keyboard and you can choose to use a Uppercase or Lowercase format, so far it's correct the it should accept it .
Check link:
https://bitcointalk.org/index.php?topic=5496335.msg64073586#msg64073586.