This is one of those annoying updates that actually make sense.

YouTube has added new requirements for external links in YouTube videos. To add an external link to your website, Patreon, Crowdfunding Sites, etc you will now have to be part of the YouTube Partner Program. This will give you the ability to add end cards to your videos which will allow for external links.

These links are really common amongst YouTubers and often you will see end cards with a link to donate money on Patreon etc. This update will mean that this ability is limited to channels that have over 10,000 subscriptions (a requirement to join the Partner Programme).

The reason this update makes sense is that YouTube needs to ensure the use of end cards does not get abused. By restricting it to the Partner Program, there will be a manual review of the channel to ensure they are not dodgy.

Here is the official word from YouTube:

In order to use externally-linking end cards in videos, creators are being asked to join the YouTube Partner program, so that we can evaluate the validity of the channel, as well as determine whether the channel is following our community guidelines and advertiser policies. This update is meant to curb abuse and does not affect current YouTube partners or existing end cards. Additionally channels do not have to actually monetize any videos as part of this requirement.