I may have found relative simple way to retain ABI compatibility, but using recent unbound code. It is not yet properly tested, but candidate change on upstream main branch is pushed to github:

https://github.com/InfrastructureServices/unbound/tree/rhel8-no-abi-break

Prepared copr build for epel8 on COPR [1]. Infrastructure is broken, so I am unsure it works as it should. But basic things seems to be working.
The change on top of released version 1.5.0 is in own branch [2]

1. https://copr.fedorainfracloud.org/coprs/pemensik/unbound/
2. https://github.com/InfrastructureServices/unbound/tree/rhel8-no-abi-break-1.15

The change looks good, even previously compiled unbound-host is able to continue with a new library and works.

But at least libreswan would require a change too, because it expects changed callback format. But that checks only version and our version would not compile libreswan.

Fail at:
https://download.copr.fedorainfracloud.org/results/pemensik/unbound/epel-8-x86_64/04373948-libreswan/builder-live.log.gz

cc -DTimeZoneOffset=timezone -pthread -std=gnu99 -g -Werror -Wno-missing-field-initializers -Wno-lto-type-mismatch -Wno-maybe-uninitialized -Wall -Wextra -Wformat -Wformat-nonliteral -Wformat-security -Wundef -Wmissing-declarations -Wredundant-decls -Wnested-externs -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fstack-protector-all -fno-strict-aliasing -fPIE -DPIE -DNSS_IPSEC_PROFILE -DXFRM_LIFETIME_DEFAULT=30 -DUSE_IKEv1 -DKERNEL_XFRM -DUSE_XFRM_INTERFACE -DUSE_DNSSEC -DDEFAULT_DNSSEC_ROOTKEY_FILE=\""/var/lib/unbound/root.key"\" -DHAVE_LABELED_IPSEC -DHAVE_SECCOMP -DLIBCURL -DUSE_LINUX_AUDIT -DUSE_SYSTEMD_WATCHDOG -DLIBLDAP -DHAVE_NM -DUSE_PAM_AUTH -DUSE_3DES -DUSE_AES -DUSE_CAMELLIA -DUSE_CHACHA -DUSE_DH31 -DUSE_MD5 -DUSE_SHA1 -DUSE_SHA2 -DUSE_PRF_AES_XCBC -DUSE_NSS_KDF -DDEFAULT_RUNDIR=\"/run/pluto\" -DIPSEC_CONF=\"/etc/ipsec.conf\" -DIPSEC_CONFDDIR=\"/etc/ipsec.d\" -DIPSEC_NSSDIR=\"/var/lib/ipsec/nss\" -DIPSEC_CONFDIR=\"/etc\" -DIPSEC_EXECDIR=\"/usr/libexec/ipsec\" -DIPSEC_SBINDIR=\"/usr/sbin\" -DIPSEC_VARDIR=\"/var\" -DPOLICYGROUPSDIR=\"/etc/ipsec.d/policies\" -DIPSEC_SECRETS_FILE=\"/etc/ipsec.secrets\" -DFORCE_PR_ASSERT -DUSE_FORK=1 -DUSE_VFORK=0 -DUSE_DAEMON=0 -DUSE_PTHREAD_SETSCHEDPRIO=1 -DGCC_LINT -DHAVE_LIBCAP_NG \
	-I. -I../../OBJ.linux.x86_64.f339a8b5d56c49dda351b3e4a108cf33/programs/pluto -I../../include -I/usr/include/nss3 -I/usr/include/nspr4  -I/builddir/build/BUILD/libreswan-4.6/programs/pluto/linux-copy \
	-DHERE_FILENAME=\"programs/pluto/ikev1.c\" \
	 \
	-MF ../../OBJ.linux.x86_64.f339a8b5d56c49dda351b3e4a108cf33/programs/pluto/ikev1.d \
	-MP -MMD -MT ikev1.o \
	-o ../../OBJ.linux.x86_64.f339a8b5d56c49dda351b3e4a108cf33/programs/pluto/ikev1.o \
	-c /builddir/build/BUILD/libreswan-4.6/programs/pluto/ikev1.c
/builddir/build/BUILD/libreswan-4.6/programs/pluto/ikev2_ipseckey_dnsr.c: In function 'dns_qry_start':
/builddir/build/BUILD/libreswan-4.6/programs/pluto/ikev2_ipseckey_dnsr.c:471:27: error: passing argument 6 of 'ub_resolve_event' from incompatible pointer type [-Werror=incompatible-pointer-types]
       dnsr->qclass, dnsr, ipseckey_ub_cb, &dnsr->ub_async_id);
                           ^~~~~~~~~~~~~~
In file included from /builddir/build/BUILD/libreswan-4.6/programs/pluto/ikev2_ipseckey_dnsr.c:31:
/usr/include/unbound-event.h:258:52: note: expected 'ub_event_callback_type' {aka 'void (*)(void *, int,  void *, int,  int,  char *)'} but argument is of type 'void (*)(void *, int,  void *, int,  int,  char *, int)'
  int rrclass, void* mydata, ub_event_callback_type callback,
                             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

Breaking of libreswan can be avoided by faking 1.7.x version in unbound-event.h.  That should also fix any other potential issues with compilation time detection. ub_version() would still report correct version of unbound. But version taken from header would report 1.7.1150 for 1.15.0 release.

Thread about this at unbound-users mailing list:
https://lists.nlnetlabs.nl/pipermail/unbound-users/2022-May/007737.html

I am not proposing this change to normal unbound, therefore it would be downstream only change. No Merge request would be created at upstream. Not included in RHEL9+ or Fedora. Should would also for RHEL7, but no production build would be done for it.

It seems most of software uses just ub_resolve_async, which should be unaffected by this change.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: unbound security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:7622