I spent 20 years in the software industry where…
As I was taught, it implies that the talker doesn’t work in this software industry now. Otherwise he would have said “I have spent 20 years…”
My question is why you can’t use Present Perfect with a time period to bring a fact to the present existence, the fact about some action that stopped in the past?
I can say “I have worked in this industry”—it may mean that it is a fact that I spent some time in that industry before. But with a specified time period I just add the time prolongation to that fact. It is like the government issues a law that anybody who spent in some industry 20 years, receives a compensation. And you say—”I have spent 20 years there. Give me my money.”
As OP has correctly understood, I worked 20 years in the software industry strongly implies that you no longer work in that industry.
But that doesn’t mean I have worked 20 years in the software industry strongly implies that you still work in that industry. Perhaps you do, perhaps you don’t. Without further context, it’s meaningless to discuss whether there’s any “weak implication”).
If you want to strongly imply you’re still doing it, use I have been working in the industry for 20 years.