Many systems are vastly more complex than 20 years ago. Even just system integration is a complex web.
It's our duty to train our employees. I disagree with the philosophy that hires someone with <5 years in the field just to throw tickets at them like they are fully trained. I've seen so many talented engineers wither away and quit from this lack of mentorship. It's disrespectful to them.
If you are a tech lead, it is your duty to spend the majority of your time training.
If there are high level tasks that need to be done that only you can do, have a Jr team member to it. They obviously cannot, and will need to research the subject, learn the area, and then you can use daily review and pairing to ensure they complete it successfully. I'd prefer to have a Jr dev take a 3x to accomplish a Sr level task like SSO integration while learning the area deeply than they fiddle with some CSS while I do the SSO integration.
This builds resiliency in the team. Now two people deeply understand the SSO integration. At the same time another Jr dev maybe was adding our first message queue for job processing. Again this is far too difficult for a Jr dev, but they are getting little bits of feedback throughout the process, I'm getting the message queue we need, and the team now has two people who deeply understand it, instead of one.
If I'm on vacation, my entire team should be able to handle any issues, because they built it all.