Hmm that is strange. I just booted by RPi and checked the version and installed it without any problems:
[indi@indiberry ~]$ lsb_release -a && apt info libev-dev && sudo apt install -y libev-dev
No LSB modules are available.
Distributor ID: Raspbian
Description: Raspbian GNU/Linux 10 (buster)
Maintainer: Boyuan Yang <firstname.lastname@example.org>
Installed-Size: 214 kB
Depends: libev4 (= 1:4.25-1)
Download-Size: 125 kB
APT-Sources: http://raspbian.raspberrypi.org/raspbian buster/main armhf Packages
Description: static library, header files, and docs for libev
Static library, header files, and documentation for libev.
libev provides a full-featured and high-performance event loop that is
loosely modelled after libevent. It includes relative timers, absolute
timers with customized rescheduling, synchronous signals, process status
change events, event watchers dealing with the event loop itself, file
watchers, and even limited support for fork events. It uses a priority
queue to manage timers and uses arrays as fundamental data structure. It
has no artificial limitations on the number of watchers waiting for the
libev supports select, poll, epoll, kqueue, and inotify.
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
The following NEW packages will be installed:
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 160 kB of archives.
After this operation, 292 kB of additional disk space will be used.
Get:1 http://ftp.halifax.rwth-aachen.de/raspbian/raspbian buster/main armhf libev4 armhf 1:4.25-1 [34.5 kB]
Get:2 http://mirror.de.leaseweb.net/raspbian/raspbian buster/main armhf libev-dev armhf 1:4.25-1 [125 kB]
Fetched 160 kB in 0s (357 kB/s)
Selecting previously unselected package libev4:armhf.
(Reading database ... 289593 files and directories currently installed.)
Preparing to unpack .../libev4_1%3a4.25-1_armhf.deb ...
Unpacking libev4:armhf (1:4.25-1) ...
Selecting previously unselected package libev-dev.
Preparing to unpack .../libev-dev_1%3a4.25-1_armhf.deb ...
Unpacking libev-dev (1:4.25-1) ...
Setting up libev4:armhf (1:4.25-1) ...
Setting up libev-dev (1:4.25-1) ...
Processing triggers for man-db (2.8.5-2) ...
Processing triggers for libc-bin (2.28-10+rpt2+rpi1) ...