Traditionally, the altars (vedis वेदि) of yajnas are called citi's (चिति). This is derived from the dhatu चिञ् चयने i.e. "to collect" or "to arrange". The vedis are arranged in layers of bricks (iṣṭaka).
This itself shows that altars are supposed to be built new for each new yajna, from bricks of various shapes and sizes, arranged in various forms according to the particular yajna. The "garbha" (womb) of Agni is below the ground, which is again formed in various shapes. The three principal Agnis are consecrated in three differently shaped altars - circular (gārhapatya), square (āhavanīya) and semi-circular (dakṣiṇa).
A famous example is the Agnicayana (अग्निचयनम्) which is a type of Atirātra Soma yajna. The altar is called śyenaciti (श्येनचिति) because it is in the shape of a falcon with wings spread out.
You can see a detailed description of the Agnicayana and the śyenaciti in this Atiratra performance:
Of course, the Brahmana texts contain very detailed instructions and descriptions of various types of citi.
However, to give an easy scriptural reference, we can look at Katha Upanishad, where Yama teaches Nachiketa the secret yajna which was later named after Nachiketa. In Katha 1.1.15:
लोकादिमग्निं तमुवाच तस्मै या इष्टका यावतीर्वा यथा वा ।
He taught him the yajna which is the origin of the world, by specifying which type of bricks to be used, how many, and how (to arrange them).
So a critical part of the yajna is to build a vedi from beginning. This is not possible if the ready-made iron "vessel" is used as vedi.
The traditional vedis were always built out of bricks, and after the yajna is done, the vedi is also disassembled. The only remnant of the entire yajna is the charred remains and earth from the Agnigarbha i.e. the pit where the Agni was consecrated.
Hence it is correct to say that yajna vedi cannot be made of iron or any material in a way that it is reused again and again. The Agni needs to be installed in the earth, which is not possible in a vedi made of iron or any other material in the form of a vessel.
(Incidentally, the word for "brick" in many Indian languages is derived from "iṣṭaka". For example, Hindi "ईंठ", Kannada "इट्टिगे" etc.)