I would say it depends primarily on:
1) How long you trianed the lift previously
2) How long you stopped doing it
3) How similar is what you've been doing instead
I'm currently doing a linear progression to build back strength in squat, deadlift, press, and bench press after surgery. I'm 8 weeks post-op today, and currently doing my working sets at about 60% of previous weights. I'm working out 3x/week and progressing well. I feel like it's a GREAT opportunity to focus on form while the weight is not heavy, and it's providing just the right stimulation to work back up. At the same time I'm able to work back up relatively quickly, adding 5-10 lbs per session on each lift for my working sets of 3 sets of 5 reps.
So looking at the big picture -- the only time losing strength in a given lift matters is if you NEED max strength in any particular moment. If you don't need to be at your strongest in that particular movement, you can train other things (or take a break, or have surgery, or recover from an injury), and then you can always cycle back around to it in the training process. It will come back within a few weeks or months if you work back to it intelligently.
A lift at which you are proficient becomes stale. You drop it for a mesocycle or two but do not neglect the movement pattern entirely eg you drop OHP and do handstand push ups and dumbbell presses.
I actually did this earlier this year -- ceased the barbell pressing but did kettlebell presses, handstand pushups, etc. to prep for SFB and SFG-recert. I lost maybe 5-10% on the OHP but could probably have had that back in 3 weeks. I might even have then added more strength due to the variety work, but I didn't pursue that enough to know for sure.
