How to select part of the text of a link in Safari 14.0 on macOS 10.15.7
Not the most eloquent way of doing it but still delivers results without any extension.
Hover over link, open contextual menu (two finger press), select 'Inspect Element' (this will open split window and land you on selected element),

double click on text between span tags (to make text field 'active'), copy needed part of text, close split window.