You may not want grab bars today. You may also prefer not to reopen the shower wall to add them later. Those positions are entirely compatible.
Plan likely use, not a random timber offcut
Discuss the intended users, transfers, and likely support locations with the appropriate designer or accessibility professional. Reinforcement should be coordinated with the future hardware and the wall assembly.
Accessibility Standards Canada's accessible-ready housing guidance discusses wall reinforcement for future adaptations. It is a planning reference, not a statement that every provision is mandatory in an ordinary Ontario renovation.
Keep the waterproofing detail intact
Ask how future fasteners can be installed with suitable support and compatible waterproofing treatment. Backing does not remove the need to follow the selected grab bar's instructions or the shower system's requirements.
Leave a record someone can use
- Photographs with a scale or clear measured references.
- Backing dimensions and its relationship to finished surfaces.
- The wall and waterproofing assembly details.
- Any hardware or installation assumptions used in the design.
Do not rely on a phone photograph with no dimensions and the memory that the timber was somewhere near the niche.
We would spend a little design effort on future usefulness while access is easy. The room can look exactly as considered today while being less difficult to adapt tomorrow. Preparation is a better compromise than another decorative towel rail being asked to act as a support.