You generally can't backup the TPM key as most TPMs are designed to prevent key material extraction.
However, with LUKS there are two keys. The key slot key that is stored in the TPM is not able to be retrieved (by design) however the disk encryption key is not stored in the TPM, it is stored encrypted in each key slot. As long as you have access to the disk encryption key via an existing key slot you can create additional key slots without TPM protection. Once you have a non-TPM key slot you can transfer the drive anywhere and unlock it using that slot instead of the TPM. Of course this slot will not be protected from brute-forcing by the TPM but if using a sufficiently long passphrase for backup or transfer it should be fine.
TL;DR if you have access to the TPM you can migrate away from it. But if the TPM is your only form of access and you lose access (stolen, wiped, forget password...) then your data is irretrievable.
However, with LUKS there are two keys. The key slot key that is stored in the TPM is not able to be retrieved (by design) however the disk encryption key is not stored in the TPM, it is stored encrypted in each key slot. As long as you have access to the disk encryption key via an existing key slot you can create additional key slots without TPM protection. Once you have a non-TPM key slot you can transfer the drive anywhere and unlock it using that slot instead of the TPM. Of course this slot will not be protected from brute-forcing by the TPM but if using a sufficiently long passphrase for backup or transfer it should be fine.
TL;DR if you have access to the TPM you can migrate away from it. But if the TPM is your only form of access and you lose access (stolen, wiped, forget password...) then your data is irretrievable.