Why is there missing numbers among BIPs?

[Coding Enthusiast]

Recently I've started going through BIPs from #1 to implement whatever I've missed and I realized there are many numbers missing. I am wondering whether they were never assigned or were there BIPs that got deleted. If they were deleted then what was the reason since we already have many BIPs that were either withdrawn, rejected or replaced by newer versions but they are not removed?

Here are some of them from the start:
3 to 7
24 to 29
46
48
51 to 59
76 to 78
[..more..]
A jump to 300s

[gmaxwell]

They've been assigned in blocks in an effort to keep related BIPs together.

Also, sometimes people have ignored the process and self-assigned numbers and started using them in communications-- sometimes multiple people with the same number, when that happens the number gets temporarily skipped for assignment to avoid adding confusion. But primarily just due to grouping.

BIPs from #1 to implement whatever I've missed

You absolutely shouldn't do that. The BIP process has virtually no editorial control-- it's just a publication numbering scheme that assigns a number to anything that persists long enough requesting one.  There are many low quality / broken BIPs that no sane party should use.

Essentially the only editorial lever in BIPs is that if many people dislike a proposal they'll encourage the proposer to abandon it before it reaches the point of getting a number assigned. If the proposer can't be convinced or if no one cares enough to convince them and the proposer persists their proposal will get numbered.

[Coding Enthusiast]

You absolutely shouldn't do that. The BIP process has virtually no editorial control-- it's just a publication numbering scheme that assigns a number to anything that persists long enough requesting one.  There are many low quality / broken BIPs that no sane party should use.

I see what you say. I am mainly reading them out of interest then only focus on those that are actually used somewhere. Like URI scheme (BIP21), Version Extended WIF (BIP178 + the original one used by a couple of Electrum versions), HD wallet BIPs,...

[khaled0111]

The BIP editor can chose any number he wants there is no rules regarding this matter.

Accordin to BIP-0001, defining what a BIP is and submitted by Amir Taaki, it is recommended to chose the next available number.

Assign a BIP number (almost always just the next available number, but sometimes it's a special/joke number, like 666 or 3141) in the pull request comments.