Questions about other CryptoNote coins are on topic.
Bikercoin apparently is a CryptoNote coin so should be considered on topic, although I think the question should be worded in a better manner to narrow its focus.
Since SDC uses ring signatures, it may be on topic if the questions focuses on ring signatures or a ring signature related comparison with Monero. General SDC questions (which is a PoS coin based on the Bitcoin blockchain, not a PoW based CryptoNote currency like Monero) are clearly off topic.
Comparisons with other major coins with a SE presence (such as BTC and ETH) are also likely to continue. I closed the ETH question you cited in your question not because of ETH but because the question was far too broad. Comparison questions should be as narrowly focused as possible.
Those focused on privacy are likely to come here with technical questions related to how Monero and Zcash compare. Zcash comparison (with Monero) questions should also be allowed provided they are narrowly focused enough to make an objective answer possible. General Zcash questions (not related to Monero) clearly should be off topic.