Your frame is perfectly fine until the instant it isn't.
That instant will be when the frame is under the most stress laterally, which is during high-speed/high energy changes, like turning/braking at speed or an impact with a stone/pothole. At that point the dent becomes a focal point, and will form the apex of a fold.
My gut feeling is that the tube will push to your left, with the existing dent in the "armpit" and you will likely fall. Remember the edges of the fold have suffered a crease, and are now work-hardened and permanently weakened.
Whether this failure comes on your very next ride or in ten years is impossible to say.
The only defence is to examine the dent before and during EVERY ride and if it shows any change or deterioration then expect imminent failure. HOWEVER there is no guarantee that the dent will show any change until it fails.
Your best and safest option is to scrap the frame, and move all the components over to another frame of similar tech levels. Shop around and see what's available on the local used market, or ask your LBS in case they have anything in stock.
Ideally you want a frame that uses components of a similar age, and will let you move your fork over. OR get a replacement frame that comes with a suitable fork already.
Example - here's a mid-2000s chainset and some 2010 wheels on a 1980s frame:
This frame died due to old age (you can see the paint missing on the seat stay - it was dented there. Most components were moved over to a mid-90's cannondale frame which I purchased retail:
The handlebars and stem and saddle moved over, but I needed a different seat post. Wheels and transmission and crankset moved over along with the BB cartridge, but I needed different brake calipers.
Even the cheap bottle cages moved over, but the mudguards didn't due to lack of clearance.