Is it respectful to actively encourage other people to contribute code to a project by pointing them to a set of getting involved instructions for that project which encourages them to write patches and code without making any mention at all that a copyright assignement is required before those patches and code contributions can be merged into the mainline development branch?
I don't think that is very respectful. It's certainly not intellectually honest. Personally I would think you would want to be as upfront about legal requirements for contribution as you can in an effort to be as honest and transparent with the contributors you are recruiting to help. It would seem far worse to me to have someone spend time on code and then tell them after they have produced it that they must sign a contract before it can be included. That seems like a manipulative strong arming tactic to me. Get the contributor to do the work and then tell them about the legal strings attached to having the work be considered for inclusion.
If we are going to have a larger conversation about what respect in the FOSS ecosystem looks like, we need to make sure that conversation includes the topic of what is fair and equitable notice with regard to legal requirements when recruiting contributors.