Not necessarily scamcoins, shitcoins are called so because they don't bring anything new to the cryptocurrency table. By copycatting existing coins and lacking a clear purpose, they also lack long-term sustainability.
As of July 2021, over 6,000 cryptocurrencies have been created. Yet, most people have heard of only half a dozen - Bitcoin (BTC), Ethereum (ETH), Cardano (ADA), Dogecoin (DOGE), Monero (XMR), Litecoin (LTC). This means there are thousands of redundant coins that don't offer any new functionality.
Number of cryptocurrencies launched every year, from 2013 to 2021 | Image Source: Statista
However, shitcoins have the same appeal as scamcoins. Both rely on making people believe that an early cheap price will lead to over 10X return on investments. After all, such a rapid rise in value is no longer possible with cryptocurrencies that have large market caps, such as Bitcoin (BTC) or Ethereum (ETH).
Moreover, even popular cryptocurrencies can be designated as shitcoins. One of the most prominent ones is Dogecoin (DOGE).
Dogecoin is neither deflationary, nor it is suitable to be used as a payment cryptocurrency. For example, Bitcoin is forever limited to 21 million coins, making it a deflationary cryptocurrency akin to digital gold. In contrast, DOGE has no such limitation, as 5 billion coins are minted into circulation annually.
This makes it an inflationary cryptocurrency, except it loses value more rapidly than USD. On top of that, DOGE's transactions are costly compared to both Litecoin (LTC) and Bitcoin Cash (BCH). The latter is a hard fork of Bitcoin specifically designed for cheap daily transactions.
Therefore, DOGE's existence outside of memetics is redundant. This is one of the key properties of shitcoins. Although in the case of DOGE, its developers are known (Palmer and Markus), another telling property of shitcoins is that their developers are anonymous. Likewise, shitcoins tend to be copied from existing projects.
By their own words, Palmer and Markus created DOGE over a weekend by tweaking Litecoin's code. Lastly, you will know shitcoins by their roadmap. It will lack any specific goals and functionalities that are not already fulfilled by previous coins.