Well I looked through various guidelines relating to sololearn and technically no its not
I say if youre promoting a game, book, art piece, and similar that people have a CHOICE to buy its fine
And as mirielle said in your other question, some mods tweak the rules a bit to fit a certain situation so idk for sure.
A negative would be to put a link that a person gets money for every X clicks because those links usually lead to sites with malicious plugins, apps, executables, etc. and I believe someone from SL staff (Shahane) stated they did not like people using SL for their personal monetary gain.
I believe it wouldn't be an issue if you are a frequent user and you occasionally post a website or share your software; after all, why do we even have the feed?
But, on the other hand, if you would come here just for the sole purpose of advertising and nothing else, well, that would be an idiotic thing to do and would/could get you banned.
At the end of the day, you must understand we can't control everyone; people will always find a way to do what they want.