It can instead be linked to Special:UserLogout which will display a confirm page. Note clicking the logout link in personal toolbar send a POST request which will log out user directly (cf T25227/geerit 504565 which introduced this confirm page and gerrit 506386 which turns logout link to POST request).
T357484: Implement Logout Confirmation will add a preference to allow user to go to confirm page instead of log out directly when clicking logout link. We can reuse it, just require all temp users to use confirm page. Currently we already have a custom confirm message in Special:UserLogout for temp account so we don't need to make a new one.

In T378806#10284061, @Bugreporter wrote:

It can instead be linked to Special:UserLogout which will display a confirm page. Note clicking the logout link in personal toolbar send a POST request which will log out user directly (cf T25227/geerit 504565 which introduced this confirm page and gerrit 506386 which turns logout link to POST request).
T357484: Implement Logout Confirmation will add a preference to allow user to go to confirm page instead of log out directly when clicking logout link. We can reuse it, just require all temp users to use confirm page. Currently we already have a custom confirm message in Special:UserLogout for temp account so we don't need to make a new one.

We are already linking to Special:UserLogout. We just need to disable the code that uses JS to log the user out, AFAICT. As @Bugreporter notes, we made some changes to the confirm message in T374519: When a temporary user clicks on "Exit session" the resulting success page says they "are now logged out". I think what we need to do is allow the user to continue to Special:UserLogout if they have a temporary account, rather than invoking the JS based logout method.