I suspect it would be better worded as 'least urgent' project. Some of the most interesting projects I've done have been long-term things that didn't need to make it out for 6 or more months, and really only needed a month to implement - but they did really need to make it out at the end of those 6 months.
With things like that, though, you can a) pull the engineer off for truly urgent 'done by Friday or we lose the customer(s)' issues, and b) losing some time due to interruptions isn't really world ending. For (a) to work you'd also probably want to stop having them interrupted.
That was how I was treated for the last year or so. It's not good for morale or retention. Even if I'm working on something that doesn't have a deadline, I still want to get work done every day. I'm not tech support and I won't stay in a position where that's my job.
With things like that, though, you can a) pull the engineer off for truly urgent 'done by Friday or we lose the customer(s)' issues, and b) losing some time due to interruptions isn't really world ending. For (a) to work you'd also probably want to stop having them interrupted.