As a prolific user of Google Maps and Waze for walking, public transport, and driving instructions, this was a concern I held about the Moto G5 Plus too. I made the decision to purchase the phone, and do not have any regrets.
Even when following walking directions, I do not find the lack of magnetometer to be an issue, and turn by turn navigation works perfectly. The only downside is that the directional arrow displayed on the main map view does not necessarily represent the direction in which you are facing. On reflection, this really is a non-issue. My previous phone was equipped with a magnetometer, though this was often miss-calibrated and gave erroneous readings.