The old forum isn’t currently read-only – @Chrisir has been pointing people here when they make new posts.
A banner on the old forum as you suggest is a good idea – I’m not sure who has access to make those changes on the old forum – @fjenett or @lmccart might know.
I think the current setup is allowing low frequency users with active accounts to be caught and redirected when they revisit old problems – or just when they revisit anything at all.
Is it hard to change the CSS? This would hide the login buttons:
.SignInPopup, .Box.GuestBox {
display: none;
}
And this
.LogoImg::after {
content: "Find the new forum at discourse.processing.org";
color: #0c2033;
background-color: #FE6CE3;
margin-left: 40px;
padding: 5px;
}