As has been said, slope angle is but one characteristic of terrain, and without actually getting eyes on the terrain in question, angle alone offers very little information on whether or not terrain can be "safely" and/or successfully navigated.
Regarding slope angle, 0 degrees is flat, 90 degrees is dead vertical. Regarding perceived slope angle, Most people think that 75 degrees is vertical and 90 degrees is overhanging. That is to say most people without a lot of mountaineering and/or climbing experience dramatically overestimate how steep terrain is.
When trip planning and using maps and e-sources with topo/relief data, I take note of areas of steep relief, particularly in places like a pass or ridge I might want to cross, as areas of potential concern but that is about it. Steep terrain often has paths of least resistance or weaknesses that can allow easy passage, and easier terrain often has short, steep, impassable sections that can be reflected on relief maps even with relatively small (100', 500' etc.) contour intervals. If I see short sections of steeper terrain on a map, say less than 1000" vertical feet I am rarely worried about it. Steep terrain for several thousand vertical feet or more I start to think about that as a substantial obstacle and/or not reasonably navigable.
Subjective factors like acceptance of risk and/or individual ability and experience play huge roles as well. What is "safe" and "passable" for one person might be high risk and/or impassable for another person with different experience and abilities. Current conditions also need to be considered. Lots of high consequence terrain in the mountains can be safely navigated when it is dry, but if it rains or snows, it could become impassable in short order.
Because of all of these subjective factors and other considerations, terrain is rated in classes that can generally be described as follows:
Class 1 - Flat, level ground
Class 2 - A normal hiking trail, uneven ground, gentle ups and downs
Class 3 - Scrambling where hands may be required but a fall will generally not lead to serious injury or death
Class 4 - Steep scrambling where a fall would likely lead to serious injury or death, climbing equipment is usually not needed or employed.
Class 5 - Rock climbing where falls will almost certainly lead to death, climbing gear/ropes is the norm
Class 6 - Climbing too steep to accomplish without the use of specialized gear for upward progress, aka big wall aid climbing.
In my experience, most hunters and hikers without mountaineering and climbing experience will dabble in class 3 terrain and on slopes up to about 45-50 degrees and that is usually about the end of their comfort level and for good reason. Getting steeper and/or more technical than that starts to open the door to serious consequences.