When someone sends me an email with several unrelated topics or tasks, there is a reasonable chance that I will miss some of them. (This is more important for tasks than for topics.)
I look at the subject line, and once I have dealt with that subject, I tend to stop reading.
If I do happen to notice that there are five tasks and can quickly do three of them, I have to do work to record the other two. If there was only one task and I wasn’t going to do it immediately I could leave it in my inbox for later, or forward it to an assistant, or just insert a link to the email into my to-do list.
(This latter ability is a function I get in Superhuman; I had not previously seen it and have found it useful with my use of Notion. I’m not affiliated with them, I’m just a grateful fan. In fact, today’s gratitude and baby animal face are dedicated to whoever at Superhuman came up with the idea.)
With single-topic emails, you can use a good subject line which makes it easier for me to find things later.
Yes, if I had a perfect GTD based workflow, the multiple topic emails would cause me less trouble. Not zero, but less.
And while my workflow is far from perfect, most people’s are much worse.
Make it easy for people to spend their time responding to your request rather than trying to track, understand and remember it.
Single topic emails are your friend. Doubly so when they carry informative subject lines.