Does the attribution removal on a sound stay with the sound if I don’t renew my Gold membership?

Yes, any sounds you download while your account is Gold retain the attribution removal privilege for life. So you (and you only) can continue to use those sounds in future projects without attribution (but you’re still subject to the other terms in the License Agreement). If you cancel your monthly or yearly donation and it returns to Basic, you are required to upgrade again if you want attribution removal and access to the wav files for any sounds downloaded and used thereafter.