A block that originated in an older version of Bitcoin and has no known parent blocks would not be a part of the main blockchain. Such a block is known as an orphan block, or a stale or detached block.
Understanding the term
It is important to note that all orphan blocks are valid but not a part of the main blockchain because such blocks have the same timestamp, which points to the main blockchain. The blockchain assumes the longer chain as valid, then the block in the shorter chain becomes an orphan. Having the same timestamp, a blockchain rejects one block but adds the other one instead.
Avoiding propagation delays on a decentralized platform is the best way of avoiding orphan blocks.
Several factors could result in orphaned blocks. For example, when two miners find a block simultaneously and thus cause a certain time-lapse during its propagation. Blocks mined under such circumstances are observed by different nodes and at different times. As nodes are required to add only valid blocks to the chain, new blocks will soon follow suit.