URLs in Newsletter
Settings / URLs and SEO / Redirect invalid case URLs to their correct versions
There is an option "Redirect all requests to lower case URLs"
This is the best setting for SEO and it is fine until now.
PROBLEM is with newsletters. It would be fine to create an exception for them. E.g. checkbox DO NOT USE THIS RULE IN NEWSLETTERS
If you put the link (e.g. Google Sheet, that uses lower and upper cases) into newsletter´s body, all characters in the link are redirect to lower case.
All is fine in Draft, but if you send it to subscribers, then the URL is changed to lower case and doesn´t work.
Subscriber cannot open the link from email body from his email client.
You can find then the wrong changed lower-case-URL in database.
(if you want to repair it, change the URL in database. But it would be
fine to prevent this situation before it happens)
Maybe you can find more situation, where lower case redirect is not ok.
Luděk Škaroupka commented
I know about the URL shorteners, but not all are ok. Google Shortener, bit.ly generate also upper case, tinyulr.com is fine.