I tried this with my site but it didn't work. It's using v7.1, but for some reason Squarespace doesn't want me to know which template I chose, and I can't remember. I can't find the disable Ajax control in site styles either. The url is wilsonwillis.squarespace.com – but the blog pages are disabled for now as they are not ready for public view. Should this script still work for me? Sorry I may not have given enough information for this query...
UPDATE: I realised that I wanted a script to apply to a summary block, not a blog post page. So the scripts here didn't work. I found a script here that did...
https://nestingzone.com/design-blog/how-limit-blog-post-excerpt-word-count