Many landlords work in either a job or self employment alongside their rental business. Some don’t however. The issue with not “working” is that you are unlikely to be paying any national insurance.
National insurance is essentially the way to get a state pension. You need sufficient qualifying years of payments to qualify. There are other ways (carers will get credit while looking after children for instance) but many don;t realise that you can pay voluntary payments of national insurance to catch up on this missing years.
A good place to start is by getting a state pension forecast by going over here!
It’s pretty painless and will give you an indication of what your position is. From there you can correct any errors (HMRC and employers don’t always report and retain the information needed) or take a view on whether it is work catching up.
The decision to make up any shortfall is a financial planning one and any financial planner worth their salt should be able to help with that.