The first output of transaction b53e3bc5…5377f141 has the following
OP_PUSHNUM_1 OP_PUSHBYTES_32 01010101…0101010101
The segwit softfork in 2017 defined versioning rules for all native segwit outputs, but only defined spending rules for version 0.¹
This output worth 5431 satoshis matches the native segwit schema, but the leading
OP_PUSHNUM_1 indicates that it is a version 1 output. It shows up as “Type: UNKNOWN”, because there are currently no spending rules for version 1 outputs active on the network. Therefore, this output could be spent by anyone, as long as they get a miner to include a non-standard transaction in their block that claims the funds.
We are anticipating the activation of the Taproot softfork in November, which will give exactly this output pattern meaning (see BIP341): Pay to Taproot (P2TR) outputs will be version 1 native segwit outputs with a 32-byte witness program that consists only of a public key.
Finally, the public key 01010101…01010101 here is obviously not randomly derived from a private key, so the private key can be assumed to be unknown. The corresponding address will essentially become a “burn address” once Taproot activates and spending of funds received there must follow Taproot’s rules.
I surmise that whoever created this output in 2019 simply wanted to be the first person to create a Taproot output on the blockchain and was willing to burn a few satoshis on that.
¹There are two output types defined in version 0: Pay to Witness Public Key Hash (P2WPKH) and Pay to Witness Script Hash (P2WSH).