If you're no longer using a domain name that has been attracting visitors, keep it. Even if you don't want it any more, keep it. A client and I were the victim of an unscrupulous silver-tongued "domainer" who abused my naivety - he registered an old, unused domain name for his own personal benefit and to the detriment of my website client.
Here's how it happened.
Some years ago I was working with a team to develop their website and one of the subjects discussed was whether to use a long domain name or an abbreviated domain name, effectively an acronym. I'll use a dummy/fake/alias organisation name to illustrate my point. If this name really exists, let me know and I'll choose another!
Let's call our fictitious organisation "Alfa Bravo Charlie Zulus," which was usually referred to colloquially as ABCZ. My recommendation was that they should register ABCZ.org.uk but they (the customer) preferred the formal alfabravocharliezulus.org.uk. So that "long domain name" was registered rather than the "short domain name" ABCZ.org.uk.
As time passed, the website became increasingly popular, not least because it had scores of external links to associated websites. After a couple of years, and with personnel changes at ABCZ, I persuaded the Hon. Sec. to also register ABCZ.org.uk, and to use both domains during a transition period, with a view to migrating fully from alfabravocharliezulus.org.uk. to ABCZ.org.uk. I think you can see why!
Eventually, when the long domain name was due for renewal, the organisation decided to "let it go" as there was little chance that anybody would want to re-register it, unless they had an interest in Zulus, and were naming themselves Alfa Bravo Charlie. So the long domain was not renewed.
Some months later, a new Hon. Sec. was appointed. While building up her knowledge of the organisation, she commented that some of the images in the website would not load. And, in fact, bits of the website were missing.
This was rather odd, and tests from other computers, using many different browsers could not see the website errors that she had reported. Then I asked her "What domain are you looking at?" and she said she didn't know but was just clicking on a link from the Favorites in the organisation's office computer. This turned out to be still pointing at alfabravocharliezulus.org.uk., instead of ABCZ.org.uk. Rather strange, because the long domain should not work. It didn't exist.
Oh, yes it did. The silver-tongued domainer has found the domain was now available and, being smart enough to find out that it had earned good Google ranking, decided to register the domain and use it for his link-building business. However, not satisfied with merely (quite legally) registering a previously-used domain name, he had the gall to use and old, copyright marked, versions of ABCZ's own website! That's not legal, decent and honest. The new Hon. Sec. could not see all the images because the "archived version" of the website (from where silver-tongued domainer obtained the copy) did not contain all the required images.
There ensued a long exchange of words with the silver-tongued domainer, who eventually removed the copyrighted content and replaced it with his own. However, the ABCZ were not satisfied with that because their audience were still being confused by which was the real and which was the imposter ABCZ website.
They tried to reason with the silver-tongued domainer, to no avail. He had even boasted on one of his many 'expert' blogs that he knew how to fight off legal claims in such circumstances, even from Nominet legal department. ABCZ was not deterred. They employed lawyers, spent a lot of time (and money) getting their story right, then unleashed the solicitors onto the silver-tongued domainer. I'll omit all the gory details. The end result was that ABCZ now have both their short and long domains back under their control. Silver-tongued domainer lost.
Most people or organisations might not have the inclination or resources or stamina for such a fight, so the easiest way to avoid such a saga happening to you is, even if you don't really want that 'old' domain, especially if it had been successful, don't let it go. Spend the few quid to maintain it every two years, and keep your domain safe from unscrupulous silver-tongued domainers.