I was a Clinton delegate to the 2016 convention and I had fun reading up on the rules. All delegates are free to vote for anyone they want at the convention and this includes so-called pledged delegates or super delegates (PLEO and automatic delegates). Candidates can protect themselves by vetting delegates. Each candidate has absolute approval right over their pledged delegates. I know that I was vetted and I helped vet other Clinton delegates. At the Texas state convention in 2016, the sanders campaign removed a delegate elected by his senate district caucus because that young man would not tell a sanders campaign official that he hated Clinton. That young man was removed but got to attended the national convention as a guest of the state party.
The candidates who have withdrawn still get to approve and vet their delegates. That means that these candidates should work with the Biden campaign to select delegates who agree with that candidates choices. This means selecting hard core/real democrats who are actual members of the Democratic Party and who will do what is best for the party. I have been working with my county and state party for a very long time which is how I got elected as a delegate. Normally it takes years of hard work inside the party to qualify as a national delegate. These people are not going to do something that is against the best interests of the party.
Please note that these candidates cannot put their name into nomination without 300 delegates. I suspect that the Biden campaign will be taking steps to vet or help the withdrawned candidates vet their delegates. I am active on Lawyers for Biden and will be making some suggestions here later.