Pete, if your old Safari version is no longer able to post to your Yahoo page you may have no choice other than to use another browser. The technology of web pages continues to evolve and browsers must evolve to keep up. As you are stuck on High Sierra you may not have that option.
If you are willing to swap to a different default browser such as Brave or Firefox you can simply export your bookmarks from Safari as a HTML file to your desktop then import said file into Brave.
You will find Export Bookmarks in the Safari File Menu and Import Bookmarks in the Brave Menu.
Having said that you might want to consider how many bookmarks that will be. If you, like me, have been using Safari since inception and syncing though iCloud you may well have hundreds of bookmarks, some of which will be redundant and others that you never use.
I tend to use Safari as a repository of all my bookmarks and when I start using a new browser I simply keep Safari open in the background and when I want to go to a known site on eg. Brave, I have two choices; I can just do a search for it or, open the site in Safari, copy the URL and paste it into Brave's search bar. Once there I can save the site as a bookmark in Brave.
This gives me an opportunity to reorganise my most used sites into appropriate folders on the new browser without the clutter of old, seldom (if ever) used bookmarks.
This is of course just my way of doing things and it may not suit you.
