This may be a third-party service, however, I think what's more important is the fact it may encourage security. I'm not asking Jagex endorse them; simply make them easier to use.
There are many people (not saying majority) that use password managers and I subjectively believe this is a more secure way to use the internet, but by making it more tedious to use such tools leads us to use simpler or less secure passwords, or worse, similar passwords to what we have on other services. Without a paste option though, it pushes people from using it as it's a pain to type 20 random characters, and tempts to using quicker and less secure alternatives.
While I hope Jagex should never suffer from a breach, it is possible and it'd be better to have more complicated passwords and ones unrelated from all other sites we have accounts with which is encouraged with use of a password manager as it's easier to achieve this way.
As far as my understanding goes, there shouldn't be any harm in simply allowing a CTRL + V in the password text field. It's minimal effort for high reward.
Please let me know if you disagree with anything said.
as someone who also uses a password manager program, I agree that this would be a good feature to have.
As for the 3rd party arguments - password management programs come with their own high security features, which are necessary given what they do. They are not interacting directly with the game, and the source of the clipboard text could come from anywhere - if you are willing to risk it, you could even be copying your password from an unsecured .txt file!!!Ooh, a light at the end of the tunnel...
...Do you think I should get off the tracks?