-snip- After updating to the current version, however, it's no longer there. So either malware added the URL to my servers list, or it was at one time a legitimate network.
Updating Electrum has nothing to do with it, unfortunately.
Electrum fetches those other non-hardcoded servers from the main server that you're connected to, it happens that it's included in the list after your client queried for other servers.
I've checked the server that you've mentioned (
electrumx[dot]info) and it is offline just recently and may come back online again.
Regardless if it's false-positive or not; apart from privacy concerns if it's auto-selected as your main server,
I don't think it can do any significant harm to your machine or wallet with the version of Electrum that you'd been using.
At least keep it blocked by your AV if you trust your AV's heuristics or don't trust the server.