I want to create a custom field named “Last Modified” and I want the field to auto populate and update each time the task is modified or updated - whether the task changes, has sub-tasks added, or a comment, question, or status is updated.
Hi @anon44091284, welcome to the Asana Community Forum 
Thanks for taking the time to provide this feedback! Currently, it is not possible for a custom field to auto populate task updates as you wish.
I’ve gone ahead and moved this thread to our #productfeedback category to give you and other community members the option to vote for this feature!
I’ll loop back in on this thread if we have any updates 
Is this available now? Seems like it’s not, at least I’m not able to assign an action to a custom field.
Is there another way to have a column for ‘last update’?
Is this available yet?
It’s not, sorry. The only thing that’s now available is a “Last Modified On” date field, but it doesn’t indicate what the change was.
How do i get that last modified on field?
Display it using the “Hide” menu:
