What should you do with old ofuda?
The standard answer today is that you should return them to the jinja where you received them, where they will be burned. This answer has a long history, and indeed there is a Japanese saying likening something to an old ofuda to mean that it no longer has any use.
There have, however, been other approaches.
The Komorëbi column in the January 1st issue of Jinja Shinpō was by Matsuo Mitsuaki, a priest who also works as a curator at the Shimanë Museum of Ancient Izumo. In the column, he reports that a priest of his acquaintance brought him a standard wooden miyagata (the shrine on a kamidana), about 50 cm high, at the museum. The priest had been asked by an ujiko to dispose of it, because it had turned up in a storeroom, but the priest thought it looked old, so he brought it to the museum.
The miyagata turned out to be packed with ofuda covering a period of forty years from the late Edo to early Meiji periods, so the mid-nineteenth century. In total, there were 994 ofuda in it, and Revd Matsuo was quite impressed that the owners had managed to get them all in. The ofuda were from both jinja and Buddhist temples, and not just local ones. There were multiple examples from jinja and temples in Kyoto, which Revd Matsuo attributes to organised schemes to get the ofuda to distant adherents, and even a few from more distant places, such as Nikkō Futarasan Jinja, in Tochigi Prefecture. (He thinks those were most likely to have been brought back as souvenirs by people who had visited independently.)
This, apparently, connects to another common approach to dealing with old ofuda. This was to put them under the roof, where they would scare away evil spirits and protect against lightning. There was even a saying that, if you collected a thousand, you would be protected. (The miyagata was six short — but, on the other hand, the storeroom seems to have stood intact for almost two centuries, so maybe it worked anyway.)
I do not think that this custom is prevalent today, but I know that people do hang on to individual ofuda or omamori that are particularly significant. The oldest one I still have is the one we received at our wedding ceremony, for example, and when we made our own omamori at a Shin’yūsha workshop, we were advised (by the priests) not to return them, because we wouldn’t be able to replace them.
The diversity of Shinto practice is one of the reasons why people who know it well are a little reluctant to describe it as “a” religion. It also means that it is impossible to lay down anything as something that you must do in order to practice Shinto. If you want to collect a bundle of a thousand old ofuda in your attic, well, there’s a tradition for that.