In general I don’t save URLs for my source information for reasons as Jerry points out.
I always find a way to capture an image or copy of the source material from the site and add-it to my database. Anything other than that runs the risk of not being supported in the future.
The other issue I have come across is related to sites like Ancestry that have country specific variations - US, UK, Australia, etc - to which individual accounts are tied. In my case (UK based), it’s no use coming across a UK census source, say, on Family Search with a URL tied to Ancestry.com when I can only access it via Ancestry.co.uk. Yes, I can edit the “com” to “co.uk” and then see the source, but that soon gets very tedious. So, URLs are a problem from several or many angles.