Previously they had been autofilling links and replacing affiliate links with their code:
https://www.theverge.com/2020/6/8/21283769/brave-browser-affiliate-links-crypto-privacy-ceo-apology
They stopped doing that but I found they appear to remove affiliate links altogether unless it comes from them.
Just to test I put NordVPN's affiliate link structure and viola it gets removed:
<a href="https://go.nordvpn.net/aff_c">Test Affiliate Link</a>