YouTube is currently testing a new feature that highlights the 'Subscribe' button anytime the word 'subscribe' appears in a video. The experimental update was spotted across a wide range of content, therefore it seems that the platform itself has fully automated the procedure.

source https://www.gadgets360.com/apps/news/youtube-subscribe-button-glowing-channel-subscription-new-feature-google-4373650#rss-gadgets-all