Polite word or short phrase that means "this isn't a high enough priority for us to work on now"

Solution 1:

You should consider deferred.

ODO:

defer VERB

[WITH OBJECT]
1 Put off (an action or event) to a later time; postpone.

‘Laois County Council have deferred all meetings of Laois County Council until further notice.’

Also, under consideration.

dictionary.com:

under consideration
Being thought about or discussed, as in Your application is under consideration; we'll let you know next week.

Solution 2:

That change is on our on the product backlog.

This term is part of Agile terminology, and is no longer niche or technical.

What is a product backlog?
A product backlog is a prioritized list of work for the development team that is derived from the roadmap and its requirements. The most important items are shown at the top of the product backlog so the team knows what to deliver first. The development team doesn't work through the backlog at the product owner's pace and the product owner isn't pushing work to the development team. Instead, the development team pulls work from the product backlog as there is capacity for it, either continually (kanban) or by iteration (scrum).

• https://www.atlassian.com/agile/backlogs